I just don't see the "social desirability" theory playing out, because what Trump supporters are shy or secretive about admitting they like Trump? They tend to be loud and proud and under the impression that everyone else is too.
Maybe it’s a strategic move. Hide from the pollsters so Democrats won’t be prepared to deal with them.
In order for that to pan out, Trump supporters would actually have to answer the polls, say they didn't vote in 2016, voted 3rd party in 2016, or voted for Clinton, but will be voting for Trump come 2020. Right now, we see very few Clinton - Trump voters in polls. If they answered Trump - Trump, they're counted the same as any other Trump supporter, and if they don't answer, the pollster will still end up weighting by 2016 vote share (which if anything, likely lowers Biden's share in the polls since D turnout was bad in 2016 and early vote suggests better news turnout wise). Idk, but I just find it hard to believe enough Trump supporters are doing this to have any meaningful statistical impact on polls that isn't already being corrected
